The replacement of fossil fuel as an energy source has led to interest in identifying renewable energy sources. Grain crops and grasses are seen as potentially valuable sources of alternative energy fuels. One such source is the use of crop plants to produce ethanol. An example of one such crop plant is corn, or other grasses and grains used to produce ethanol, typically using either wet milling or dry grinding processes.
In conventional wet milling, whole kernel corn (seed) is steeped in a liquid mixture including sulfur dioxide for a period of between 24 and 36 hours to soften the materials and loosen the components of the kernel. The corn and liquid is put in a mill that grinds the corn to free the germ from the kernel. Germ can be separated from the other materials using known technologies. A unique feature of cleaned germ that is low in starch is that it floats to the top of a starchy soak slurry and can be separated from other materials. The remaining starch, protein and fiber are separated to produce pure starch. The starch is cooked and fermented, and finally distilled to produce ethanol.
In conventional dry grind ethanol production, corn seed is ground, mixed with water, cooked, fermented with yeast, and distilled. The pericarp fiber (bran) and germ do not ferment and instead at the end of the process are produced as solids. These byproduct solids are commonly dried to 10% moisture and called distillers dried grains with solubles, also referred to as DDGS.
Conventional dry milling for food uses may also occur separate from ethanol production, where grain is ground so that the pericarp and germ are separated from the endosperm. The endosperm proceeds through and can be used for any of a variety of food or beverage applications, while the pericarp and germ may or may not be separated from one another and sold as a low value livestock feed ingredient.
An important aspect of increasing the feasibility of ethanol production is to optimize use of the byproducts produced in this process. High raw material expense and costs of bringing the raw material to plants, the cost of enzymes, yeast and chemicals used in the process, and capital and labor costs are limitations on economics of ethanol production. In the dry-grind process, no distinction is made between the fermentable starch and non-fermentable components of the seed, namely the germ, fiber and protein. These components are recovered together as DDGS post fermentation and sold at a discounted price to corn due to excess market supply and poor handling characteristics. There is keen interest in improvement in purity of these components and removing them prior to ethanol fermentation, in order that they may be sold for separate higher value uses and improve the economics of the operation.
New ethanol pre-processing technologies termed “fractionation or frac” are under development. One processing technology incorporates or combines the germ and pericarp removal processes common in conventional food dry-milling with the conventional dry-grind ethanol process to grind the seed before steeping to create a higher purity starch stream to enhance ethanol efficiencies. One limitation of the dry frac for ethanol process is the loss of starch with the germ and pericarp removal which lowers the quality of the germ and pericarp while also lowering the ethanol yield per bushel of corn processed.
In corn seed a majority of the oil is concentrated in the germ tissue. Corn oil has been traditionally removed from the germ using hexane solvent extraction processes. A very large percentage of this germ is produced and extracted by the conventional corn wet milling industry. Standards for corn germ have been developed over many years based on the corn wet milling industry. Corn germ that is desirable for extraction must be greater than 35% oil dry matter basis (dmb) and less than 5.0% moisture. To date this has not been achievable with germ produced by dry milling processes. Currently germ produced in a dry milling process has about 20-25% oil content (as is basis).
TABLE 1Typical composition of Dry-Milling Byproducts Streams (as is basis)Component (%)corn germcorn branhominy feed1cornMoisture9.610.013.514.0Protein15.88.08.011.0Fat23.84.53.44.5Crude Fiber5.712.04.72.5Ash6.72.52.02.0Starch18.435.061.060.0Other polysaccharides20.028.07.46.0Alexander, 1987.1Feedstuffs. Reference Issue. 2007.
The germ that is removed using the conventional dry fractionation processes suffers from low oil concentrations, as well as high starch and moisture content. Currently, the conventional wet milling industry uses only whole kernel and discriminates against broken corn pieces. If one were to use this dry fractionated material(s) in a conventional wet milling operation, a serious complication is tank foaming during the soak or steeping phase. In addition, yield loss would be observed for oil extraction processing because of the high residual starch levels, making dry milled germ an undesirable feed stock. This is because the loose starch material restricts hexane circulation by fouling pumps. Ethanol or dry milling plants producing dry fractionated germ in this fashion are often forced to sell this material into the low value animal feed market. In contrast, high purity wet-milled germ has a value significantly higher since it is further processed into edible oil for human consumption. Additionally, the high starch content of dry fractionation germ reduces the amount of starch available for fermentation, resulting in lower ethanol plant yields and reducing profitability. S. A Matz (1991) reports that the process of extracting oil is not efficient if the germ contains ‘significant’ amounts of foreign material (eg starch). It is therefore essential to ‘carefully’ clean the germ fraction.
Further, no product to date produced from such a conventional wet or dry milling process has both high oil purity and high protein dispersiblity index or PDI. The latter measures the percentage of total protein that is soluble in water. Higher oil content of germ fractions is desirable, as is high PDI. A high PDI value improves processing yields which in turn lowers overall manufacturing costs. The corn protein extracted can be used in human consumption as a replacement for expensive animal based protein foodstuffs, beverages cosmetics or pharmaceuticals.
The pericarp that is removed by conventional dry fractionation is limited in food applications due to excess starch levels which further reduces ethanol yields. Higher corn costs that result from increased ethanol production make this problem even greater. Recent changes to the FDA Food Pyramid guidelines suggest the need for increased levels of daily fiber intake. Beyond being low in fat, rich in vitamins and minerals, corn bran is a concentrated source of dietary fiber, not starch and sugars. Refined corn bran is discussed in the literature as being insoluble and having 18% cellulose and 67% hemicellulose and a water-holding capacity of about 2.4:1 (Burge and Duensing, 1989). Food manufacturers are therefore interested in utilizing new ingredients that are high in total dietary fiber (TDF) and consistent. Conventional dry milled fiber would have issues in this regard in that it is difficult to control the amount of starch that remains attached to the fiber without adding expense of further mechanical processing or polishing. Food products containing dry milled corn bran would therefore have varying levels of starch. This in turn would deteriorate the products' consistency, manufacturing processes and increase the risk of falling outside of product nutrient label guarantees. Depending on the food application, corn bran is processed and sized to meet specific granulation requirements. Coarse granulation bran is used in extruded products and ready to eat cereals High fiber baked goods, low-calorie sports drinks and soluble mixes would use bran from a fine mesh sort.
Freeman (1973) and Wang and Eckhoff (2000) discuss that the conventional corn wet milling industry has specific needs of corn quality for processing. Processors work to minimize handling, shipping and conveying to reduce the chance of kernel breakage. Freeman points out that broken pieces of corn (eg starch germ or starch fiber pieces) must be removed by screening before processing as they interfere with the flow of the steeping medium. This is because the sloughed off starch, sugars and protein from the broken pieces enter the steepwater and cause gelling during evaporation of steepwater. In turn, gelling increases steep-liquor viscosity and restricts water flow through the steeps and screens, increasing equipment wear.
The broken pieces of corn are cleaned or aspirated off prior to entering the soak or steep process and are added back to the corn gluten feed, which is a low value feed ingredient compared to refined germ or corn gluten meal. Thus wet millers impose a high discount to discourage delivery of corn that may contain damaged or broken corn. In the book Corn: Chemistry and Technology, 2nd ed, the authors elaborate the wet millers prefer whole kernel corn which has minimal cracks and broken material. This is preferred since less starch would then wind up in the steep water.
Previously researchers have evaluated the concept of merging conventional dry milling and conventional wet milling operations to create greater efficiencies (Gillenwater et al, 1970, Powell and McGeorge and Chwalek and Olson, 1980). Germ and bran are separated from endosperm. The endosperm ‘grits’ are then put into a wet steeping process and a thus termed steeping grits. It was noted that in the conventional dry milling process, the germ gets damaged and contaminates the purity of the steeping grits. Because germ contains oil, the authors note that oil causes downstream problems. This hybrid process which attempts to combine conventional dry and wet milling processes is not practical with dry milled germ as one of the substrates. A benefit claimed by the authors of their hybrid wet-dry process is that they do not need the degermination mills, germ separators, germ dryers, and the screens and presses required to wash and de-water the germ and hulls in addition to other expensive apparatuses are eliminated. This is accomplished by removing the hull and germ from the corn kernel before the steeping step.
Attempts made to improve on conventional wet milling product recovery include that described in Slabbekoorn et al, in WO2005/074704. (This reference and all references cited are incorporated herein by reference). The process uses whole corn in a wet milling process for the purpose of concentrating corn gluten. There, corn gluten, that is, the water insoluble proteins derived from the endosperm, is used as a corn protein material produced in a wet milling process. This material is contacted with a wet mill stream produced in the wet milling process (such as corn steep liquor) along with an enzyme to aid in removing carbohydrates. This wet process produces starch liquids separate from the zein proteins, used for corn sweeteners and the like, and a concentrated corn gluten protein product. In the wet milling process, a high quality germ is already produced, and the previous attempts have been focused on improving gluten byproduct as a protein source.
In the conventional dry-milling process, as described here, removal of the zein proteins from starch is not necessary. Instead, it is the fiber, germ and/or hominy that is produced in conventional dry milling that is ordinarily a low-value product that is desired to be transformed into valuable byproducts of dry milling. Attempts to improve recovery of byproducts have included steps to recover germ separately, such as where the whole kernel is soaked prior to degermination and before proceeding with ethanol process, and/or to recover pericarp separately, by separating pericarp following soaking. See for example Singh et al., U.S. Pat. No. 6,254,914. Also see pending application U.S. Ser. No. 11/653,562, published as US20070184159 for an approach where the components are soaked and ground such that hydraulic lift is used to separate the components.
Therefore there is a need to improve the quality and purity of the byproducts of the conventional dry milling processes.